class_docstr: Callback for executing different checkpoint requirements.
code: def checkpoint(self):
      return self._checkpoint
  
file_path: mmf/mmf/trainers/callbacks/checkpoint.py
imports: import logging
  from mmf.trainers.callbacks.base import Callback
  from mmf.utils.checkpoint import Checkpoint, consolidate_optim_state_dict
label_desc: Use image node_id 2 for calling the CheckpointCallback obj's underlying member method code with example usage: obj.checkpoint() and returns: self